1. Field of the Invention
This invention relates to a system for fracing producing formations for the production of oil or gas and, more particularly, for fracing in a cemented open hole using sliding valves, which sliding valves may be selectively opened or closed according to the preference of the producer.
2. Description of the Related Art
Fracing is a method to stimulate a subterranean formation to increase the production of fluids, such as oil or natural gas. In hydraulic fracing, a fracing fluid is injected through a well bore into the formation at a pressure and flow rate at least sufficient to overcome the pressure of the reservoir and extend fractures into the formation. The fracing fluid may be of any of a number of different media, including sand and water, bauxite, foam, liquid CO2, nitrogen, etc. The fracing fluid keeps the formation from closing back upon itself when the pressure is released. The objective is for the fracing fluid to provide channels through which the formation fluids, such as oil and gas, can flow into the well bore and be produced.
One of the prior problems with earlier fracing methods is they require cementing of a casing in place and then perforating the casing at the producing zones. This in turn requires packers between various stages of the producing zone. An example of prior art that shows perforating the casing to gain access to the producing zone is shown in U.S. Pat. No. 6,446,727 to Zemlak, assigned to Schlumberger Technology Corporation. The perforating of the casing requires setting off an explosive charge in the producing zone. The explosion used to perforate the casing can many times cause damage to the formation. Plus, once the casing is perforated, then it becomes hard to isolate that particular zone and normally requires the use of packers both above and below the zone.
Another example of producing in the open hole by perforating the casing is shown in U.S. Pat. No. 5,894,888 to Wiemers. One of the problems with Wiemers is the fracing fluid is delivered over the entire production zone and you will not get concentrated pressures in preselected areas of the formation. Once the pipe is perforated, it is very hard to restore and selectively produce certain portions of the zone and not produce other portions of the zone.
When fracing with sand, sand can accumulate and block flow. United States Published Application 2004/0050551 to Jones shows fracing through perforated casing and the use of shunt tubes to give alternate flow paths. Jones does not provide a method for alternately producing different zones or stages of a formation.
One of the methods used in producing horizontal formations is to provide casing in the vertical hole almost to the horizontal zone being produced. At the bottom of the casing, either one or multiple holes extend horizontally. Also, at the bottom of the casing, a liner hanger is set with production tubing then extending into the open hole. Packers are placed between each stage of production in the open hole, with sliding valves along the production tubing opening or closing depending upon the stage being produced. An example is shown in U.S. Published Application 2003/0121663 A1 to Weng, wherein packers separate different zones to be produced with nozzles (referred to as “burst disks”) being placed along the production tubing to inject fracing fluid into the formations. However, there are disadvantages to this particular method. The fracing fluid will be delivered the entire length of the production tubing between packers. This means there will not be a concentrated high pressure fluid being delivered to a small area of the formation. Also, the packers are expensive to run and set inside of the open hole in the formation.
Applicant previously worked for Packers Plus Energy Services, Inc., which had a system similar to that shown in Weng. By visiting the Packers Plus website of www.packersplus.com, more information can be gained about Packers Plus and their products. Examples of the technology used by Packers Plus can be found in United States Published Application Nos. 2004/0129422, 2004/0118564, and 2003/0127227. Each of these published patent applications shows packers being used to separate different producing zones. However, the producing zones may be along long lengths of the production tubing, rather than in a concentrated area.
The founders of Packers Plus previously worked for Guiberson, which was acquired by Dresser Industries and later by Halliburton. The techniques used by Packers Plus were previously used by Guiberson/Dresser/Halliburton. Some examples of well completion methods by Halliburton can be found on the website of www.halliburton.com, including the various techniques they utilize. Also, the sister companies of Dresser Industries and Guiberson can be visited on the website of www.dresser.com. Examples of the Guiberson retrievable packer systems can be found on the Mesquite Oil Tool Inc. website of www.snydertex.com/mesquite/guiberson/htm.
None of the prior art known by applicant, including that of his prior employer, utilized cementing production tubing in place in the production zone with sliding valves being selectively located along the production tubing. None of the prior systems show (1) the sliding valve being selectively opened or closed, (2) the cement therearound being dissolved, and/or (3) selectively fracing with predetermined sliding valves. All of the prior systems known by applicant utilize packers between the various stages to be produced and have fracing fluid injected over a substantial distance of the production tubing in the formation, not at preselected points adjacent the sliding valves.